A Natural History of Millbrook Marsh,
A Wetland In An Urbanizing Setting

Stream Morphology, Habitat, and Discharge

In 1997, Slab Cabin Run averaged 5.5 m (18.1 ft) wide and 0.3 m (1.0 ft) deep. It consisted of approximately equal proportions of riffle, glide, and pool habitats (Table 3-14). Only 36% of the bank was rated as stable. Gravel was the predominant substrate and silt ranked second (Brooks et al. 1998). The earliest data available for Slab Cabin Run is the "Commonwealth of Pennsylvania Board of Fish Commissioners Stream Survey Report" of April 7, 1941. The "lower 2/3 mile" (1073 m) from the Thompson Run confluence to Spring Creek had a sandy bottom and scarce aquatic vegetation (Donley 1941). Slab Cabin Run was included in the study area for a thesis on the ecology of the Muskrat on Spring Creek (Smith 1954). In 1953, it was a narrow, shallow stream with a comparatively silt-free bottom until after the junction with "Willow Run", at which point it became much wider, silt laden, and 0.6 - 1.2 m deep. Other statements within the report, such as the description of the cattail-sedge marsh just above the confluence of Willow and Slab Cabin Runs, suggest that "Willow Run" is actually Thompson Run. The PennDOT study of 1980 describes Slab Cabin Run as 4.8 m (15.75 ft) wide and 0.76 m (2.49 ft) deep. The stream’s 0.6 to 1.2 m (1.96 to 3.94 ft) banks were vertical in 1980 and the substrate was a rock and gravel dominated stream bed (PennDOT 1981). In 1987, the substrate of Slab Cabin Run in Millbrook Marsh downstream of the confluence with Thompson Run was found to be "quite silted" (Bureau of Water Quality Management 1989). By 1989, the riffle substrate in Slab Cabin Run about 800 m above Thompson Run was mostly gravel mixed with sand and boulders and the rocks were about 30% embedded (Hughey 1990) (Figure 2-4, 3-5, 3-6).

In 1997, Thompson Run averaged 4.8 m wide and 0.3 m deep. Riffles made up 55% of the instream habitat and 88% of the bank was rated as stable. Gravel was the predominant substrate and cobble ranked second (Table 3-14) (Brooks et al. 1998). The earliest data for Thompson Run is also a Stream Survey Report, from April 8, 1941. The stream bottom had a fine gravel to sandy substrate and abundant aquatic vegetation (Donley 1941). In 1989, Hollender described Thompson Run as 300 m long, 4.4 m (14.4) wide, with a rubble and gravel substrate (Hollender 1989). Also in 1989, Hughey (1990) described the riffle substrate as mostly sand and fine gravel that "looked as if it had washed into the stream from the shoulder of PA 26" with the large rocks being about 40% embedded.

Table 3-14.
Summary of habitat composition in reaches of Slab Cabin Run and Thompson Run within Millbrook Marsh (Brooks et al. 1998)

             

% substrate composition

 

%
riffle

%
glide

%
pool

% stable
bank

%
bank
fish
cover

%
instream
fish
cover

boulder

cobble

gravel

sand

silt

Slab Cabin Run :
 

27

38

35

36

32

21

6

11

42

10

31

Thompson Run
 

55

35

10

88

43

17

4

26

63

2

5

The high percent of unstable banks in Slab Cabin Run found in 1997 are evidently due to high stream flows attributable to stormwater runoff (Brooks et al. 1998).

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Figure 3-5 Gravel bar formation in Thompson Run. July 1998.

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Figure 3-6 Gravel bar formation in Thompson Run. July 1998.

One reach of Slab Cabin Run, sampled in 1987, had an inordinate amount of silt as a result of extensive erosion from the mid-reaches of Slab Cabin Run. The heavily silted portion is also below the confluence with Thompson Run (Bureau of Water Quality Management 1989) which most likely contributed a substantial amount of sediments. Slab Cabin Run has a higher proportion of silt than Thompson Run. This is possibly due to a combination of the high sediment load from upstream areas in Slab Cabin Run and the higher water velocity in Thompson Run. With the velocity decreasing after entering the wider Slab Cabin Run, sediments can then settle out of the water. Bar formation is evident below the confluence (Figure 3-7, 3-8).

In June 1997 the discharge of Slab Cabin Run leaving the marsh was 0.608 cms and about 40% of this flow, 0.243 cms was from upper Slab Cabin Run (Table 3-15). Thompson Run discharge was 0.292 cms. Most of the flow in Thompson Run, which contributed about 48% of the flow leaving the marsh, originates from Thompson Spring. The remaining 7% of flow, 0.045 cms, came from Bathgate Spring and 5%, 0.028 cms, originated from smaller springs that were not measured. In October 1997, following a dry summer, the discharge of Slab Cabin Run leaving the marsh was 0.274 cms and only 13% of this water, 0.035 cms was contributed by upper Slab Cabin Run. Thompson Run contributed 66% of the flow (0.180 cms) and the remaining .059 cms, 21%, was provided by Bathgate Spring and other unmeasured springs (Brooks et al. 1998).

In July 1959, McDonnell reported the discharge of Thompson Run as 0.246 cms. Stream discharge data from October 1959 showed that the discharge of Slab Cabin Run leaving the marsh was 0.303 cms. According to McDonnell’s data, Upper Slab Cabin

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Figure 3-7 Confluence of Slab Cabin Run (left) and Thompson Run (right), looking upstream. July 1998.

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Figure 3-8 Slab Cabin Run just after Thompson Run confluence. Degraded streambank is opposite the confluence. July 1998.

Run contributed 0.082 cms and Thompson Run, 0.221cms (McDonnell 1960). Stream discharge was measured in some areas and calculated in others. The resulting data does not include Bathgate Springs or other smaller springs within Millbrook Marsh.

The discharge data illustrates the importance of ground water in maintaining the aquatic communities in the marsh. The Bathgate and smaller springs in Millbrook Marsh seemed to discharge fairly consistent amounts over the course of the 1997 growing season, for the most part independent of precipitation. In October after the dry summer, Bathgate Springs were emitting 73% and smaller springs were still emitting 92% of what they did in June (Table 3-15). Thompson Spring also remained fairly constant, especially compared to Slab Cabin Run. The flow in October of Thompson Run was 61% of the June flow, where Slab Cabin Run flow was only 14% of the June flow. Slab Cabin Run appeared to J. H. Clark (1965) as an effluent, or gaining stream only from the south edge of Lemont to the confluence with Spring Creek which includes the reaches within Millbrook Marsh. An effluent stream is one sustained by the consistent yield of springs. Slab Cabin Run has groundwater discharging into it along that length that it is an effluent stream. Above that point in Lemont, though, it is both a discharge and recharge of groundwater. At some points it disappears into sinkholes and rises again at other points. Along those lengths it alternates between a water-table effluent stream and an underdrained influent stream, depending on the season. Slab Cabin Run in those reaches above Lemont can go dry during times of drought. The flow is greatly decreased during drought times because of the lack of flow upstream. Within Millbrook Marsh it will not stop completely since it is an effluent stream in that area.

Table 3-15
1997 Millbrook Marsh Streams and Springs Discharge

 

June

October

 

cms

% of total discharge

cms

% of total discharge

% of June discharge

Thompson Run, Route 26

0.292

48

0.180

66

61

Slab Cabin Run, Route 26

0.243

40

0.035

13

14

Confluence of Bathgate Springs

0.045

7

0.033

12

73

Other Springs

0.028

5

0.026

9

92

Slab Cabin Run, Puddintown Road
(Total MM discharge)

0.608

100

0.274

100

45

Thompson Run, on the other hand, is an effluent stream along the entire length. It is fed by the sustained yield of springs, mainly Thompson Spring, and therefore, does not decrease as Slab Cabin Run does. Though the discharge from Thompson Spring is great, about half of Bellefonte’s famous Big Spring, the historical data shows a possibility of a gradually decreasing discharge. Discharge in 1941 was measured at 0.247 cms, in 1944 at 0.237 cms, in 1965 at 0.219 cms, in 1971 at 0.170 cms, and then in 1985 at 0.130 to 0.192 cms (PA Fish Commission 1941, Clark 1965, Wood 1980, Miller 1985)). The historical data is limited, though, and more in-depth research and consistent monitoring is necessary for any conclusive statement regarding Thompson Spring.

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